Apolipoprotein E (apoE) has been recently implicated in the pathogenesis of Alzheimer's disease (A.D.) One of the apoE alleles, epsilon4, behaves as an autosomal co-dominant trait in the majority of late-onset and sporadic A.D.. The apoE4 gene dose is a major risk-factor susceptibility gene for A.D. with homozygosity for this allele virtually sufficient to cause disease by age 80, and with 50% of homozygous patients developing disease by age 68. In contrast, the epsilon2 and epsilon3 alleles decrease the probability of disease, and increase the age of onset, with the protective effect of epsilon2 greater than epsilon3. Thus the inherited apoE allele determines in part, the risk of developing Alzheimer disease, and determines the rate of disease progression. Interactions of apoE protein with other molecules is therefore critical int he disease process, with isoform-specific interactions of apoE determining the probability,a nd rate, of disease expression. The three common protein isoforms of apoE; E2, E3, E4, differ from each other by one amino acid, which determines their profoundly differing interactions with other proteins. In vitro, apoE4 binds betaA peptide (the primary constituent of the neuritic plaque) faster, and with a different pH dependence, than does aplE3. This isoform-specific difference observed in vitro correlates with the greater betaA peptide amyloid burden deposited in situ in homozygous epsilon4 A.D. patients, compared with homozygous epsilon3 A.D. patients. Paired-helical filaments of the neurofibrillary tangle are composed of tau protein. ApoE3 avidly binds tau in vitro, forming a complex not dissociated by boiling in SDS. In contrast, apoE4 does not form such a complex. Isoform-specific interactions of apoE with tau cold alter tau function or metabolism. ApoE is found in populations of neurons, some of which contain neurofibrillary tangles. In vitro interactions of the apoE isoforms have been studied only semi- quantitatively under harshly denaturing conditions. The technique of surface plasmon resonance will be employed to characterize interactions of the apoE isoforms with other molecules quantitatively and under physiologic conditions. This technique permits the characterization of protein interactions including affinity measurements to 10-6M sec-1. No derivitization of the proteins is required, and the sensitivity of detection is picomolar. Quantitative characterization of isoform-specific interactions of apoE with other molecules under physiologic conditions is essential to identify molecular targets by which apoE is relevant to A.D.